I was doing a run today - nothing out of the ordinary - my usual 3 mile circuit that finishes with a half mile hill run.

Half way through I literally thought someone had pulled the duracell right out of me! I had to stop and walk home.

Its very very rare that happens to me, but the burning questions I have are:

1.) Can you fight through this or is it better to just stop

2.)Has anyone else experienced this. Dont get me wrong im not talking about fatigue here, because its something I do on a regular basis. Is it sugar, body fuel, what?

Eek, that sounds alarming although I've had that happened to me as well. Could be down to a few reasons:

1. Anxiety - you've a stressful time ahead
2. Blood sugar has taken a dive (I take you're not pre-menstrual though ha ha!)
3. It's the height of the hay fever season, I feel like I've been punched in the face today

I'd be inclined to stop when your body tells you to, if you try to force yourself through it, you'll cause yourself all manner of problems.

It sounds to me like your body went into a kind of conservation mode after your exercise shifted from aerobic and anaerobic exercise, into the level of predominantly anaerobic exercise. Maybe slowing down and breathing deeply would switch it back and you could start up with a second wind as the aerobic exercise takes back over.

My physiology is ok but my exercise physiology is almost non-existent.

I used to get this every so often when doing circuit training, particularly on hot days. As you say, it wasn't normal fatigue and it felt like someone had turned the power off.

With me, my vision would also narrow. The one time I tried to fight through it, I nearly collapsed and had to sit down. Suppose I should have listened to my body.

Someone suggested it was a blood sugar thing so whenever it happened, I would have a sugary drink and then I would be okay in ten minutes and would sometimes spar. I haven't experienced this in a long time.

Not sure if this helps.

I'm kind of doing an experiment on myself at the moment. I am training 6 days a week,mix of cardio, weights, mma and kung fu with some swimming and a fair bit of sparring too. I feel really tired today after mma yesterday and tomorrow night I have mma and kung fu. Normally this morning I have a cardio session but was cancelled today so I had a lie in and now feel really tired. I'm just heading out to the gym to see what I can do when really fatigued.I know people say you should listen to your body, but I am interested in seeing what I can push myself to do at the moment. A couple of weeks ago after my normal training week I was completely exhausted on my rest day, then last week I felt better on my rest day, so I am picking up the intensity of my training.
If you train heavily you really need to watch your fluid intake as you just won't feel like you have anything left.
I dunno, the experiment may fail and I may be left being completely exhausted, but part of me just wants to do it to see how far I can push myself and improve my strength and fitness. I also want to see how well I can fight when I am fatigued.

I had a couple of training days like this at the gym with my trainer. I suddenly just felt like I couldn't do anymore after just one rep of whatever she had me doing. Plus, I actually felt like passing out and it wasn't because I was fatigued. I think mainly it had to do with just simply not having enough in the tank and trying to push myself through my workout while maintaining the same level as I once did when I was working normal day time hours.

Don't rule out rhabdomyolysis: http://www.nlm.nih.gov/medlineplus/ency/article/000473.htm

In short, this occurs when you overexert your muscles and they break down too much, flooding your body with myoglobin, which in bad cases can cause fatal kidney failure.

If your hydration and glucose levels are good, and if you're not in a chronically overtrained state, then listen to your body. If it says stop (or, really, if it just won't go any more), then stop.

But the trouble is distinguishing that from when your body is still game but your brain is lying just to get off easy, eh?